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  MySQL5.7新特性之Generated Column
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  <p>MySQL5.7新特性之Generated Column(生成列)，<a href="https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html" target="_blank" rel="noopener">官网文档</a>。</p>
<a id="more"></a>

<h1 id="介绍"><a href="#介绍" class="headerlink" title="介绍"></a>介绍</h1><p>Generated Column是MySQL 5.7引入的新特性，所谓Cenerated Column，就是数据库中这一列由其他列计算而得，直译生成列。</p>
<p>在MySQL 5.7中，支持两种Generated Column，即Virtual Generated Column（虚拟生成的列）和Stored Generated Column（存储生成的列），二者含义如下：</p>
<ul>
<li>Virtual Generated Column（虚拟生成的列）：不存储该列值，即MySQL只是将这一列的元信息保存在数据字典中，并不会将这一列数据持久化到磁盘上，而是当读取该行时，触发触发器对该列进行计算显示。InnoDB支持Virtual Generated Column。</li>
<li>Stored Generated Column（存储生成的列）： 存储该列值，即该列值在插入或更新行时进行计算和存储。所以相对于Virtual Column列需要更多的磁盘空间，与Virtual Column相比并没有优势。因此，MySQL 5.7中，不指定Generated Column的类型，默认是Virtual Column。</li>
</ul>
<p>在表中允许Virtual Column和Stored Column的混合使用</p>
<h1 id="创建语句"><a href="#创建语句" class="headerlink" title="创建语句"></a>创建语句</h1><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">col_name data_type [GENERATED ALWAYS] AS (expr)</span><br><span class="line">  [VIRTUAL | STORED] [NOT NULL | NULL]</span><br><span class="line">  [UNIQUE [KEY]] [[PRIMARY] KEY]</span><br><span class="line">  [COMMENT &#39;string&#39;]</span><br></pre></td></tr></table></figure>



<h1 id="使用"><a href="#使用" class="headerlink" title="使用"></a>使用</h1><p>例如，知道直角三角形的两条直角边，要求斜边的长度。很明显，斜边的长度可以通过两条直角边计算而得，那么，这时候就可以在数据库中只存放直角边，斜边使用Generated Column，如下所示：</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"> </span><br><span class="line">-- 创建表</span><br><span class="line">CREATE TABLE triangle (</span><br><span class="line">  sidea DOUBLE,</span><br><span class="line">  sideb DOUBLE,</span><br><span class="line">  sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb))</span><br><span class="line">);</span><br><span class="line">-- 插入</span><br><span class="line">INSERT INTO triangle (sidea, sideb) VALUES(1,1),(3,4),(6,8);</span><br><span class="line"> </span><br><span class="line">-- 查询</span><br><span class="line">mysql&gt; SELECT * FROM triangle;</span><br><span class="line">+-------+-------+--------------------+</span><br><span class="line">| sidea | sideb | sidec              |</span><br><span class="line">+-------+-------+--------------------+</span><br><span class="line">|     1 |     1 | 1.4142135623730951 |</span><br><span class="line">|     3 |     4 |                  5 |</span><br><span class="line">|     6 |     8 |                 10 |</span><br><span class="line">+-------+-------+--------------------+</span><br></pre></td></tr></table></figure>

<p><strong>注意事项：</strong></p>
<ul>
<li>在generated column上建立索引，建立索引以后，能够加快查找速度 </li>
<li>生成的列不允许人为指定值，否则会引发ERROR 3105的错误</li>
</ul>
<p><strong>限制：</strong></p>
<p>虽然一般情况下都应该使用Virtal Generated Column，但是，目前使用Virtual Generated Column还有很多限制</p>
<ul>
<li>聚集索引不能包含virtual generated column   </li>
<li>不能在Virtual Generated Column上创建全文索引和空间索引</li>
<li>Virtual Generated Column不能作为外键</li>
<li>创建generated column(包括virtual generated column 和stored generated column)时可以使用确定性内置函数和运算符,但不能使用非确定性的（不可重复的）函数（比如NOW ）</li>
<li>子查询、参数、变量和用户定义的函数都是不允许的</li>
</ul>
<h2 id="用法示例"><a href="#用法示例" class="headerlink" title="用法示例"></a>用法示例</h2><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> <span class="keyword">test</span> (</span><br><span class="line">    <span class="keyword">id</span> <span class="built_in">INT</span> (<span class="number">11</span>) <span class="keyword">UNSIGNED</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> AUTO_INCREMENT,</span><br><span class="line">    chinese <span class="keyword">DOUBLE</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'0'</span>,</span><br><span class="line">    math <span class="keyword">DOUBLE</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'0'</span>,</span><br><span class="line">    english <span class="keyword">DOUBLE</span> <span class="keyword">NOT</span> <span class="literal">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'0'</span>,</span><br><span class="line">    total_score <span class="keyword">DOUBLE</span> <span class="keyword">AS</span> (chinese + math + english),</span><br><span class="line">    PRIMARY <span class="keyword">KEY</span> (<span class="keyword">id</span>)</span><br><span class="line">) <span class="keyword">ENGINE</span>=<span class="keyword">INNODB</span> <span class="keyword">DEFAULT</span> <span class="keyword">CHARSET</span>=utf8mb4;</span><br></pre></td></tr></table></figure>

<p>如果要在 insert 语句中包含 total_score 字段名，则只能将其值设为 DEFAULT</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">insert</span> <span class="keyword">into</span> <span class="keyword">test</span>(chinese, math, english, total_score) <span class="keyword">values</span>(<span class="number">33</span>, <span class="number">44</span>, <span class="number">55</span>, <span class="keyword">DEFAULT</span>);</span><br></pre></td></tr></table></figure>

<p>如果表已经存在了，可以通过alter table语句来创建，修改，删除生成列。</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">add</span> <span class="keyword">column</span> times_score <span class="keyword">double</span> <span class="keyword">generated</span> <span class="keyword">always</span> <span class="keyword">as</span> (chinese * math * english) <span class="keyword">stored</span>;</span><br></pre></td></tr></table></figure>

<p>修改生成列的数据类型和表达式</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">modify</span> <span class="keyword">column</span> times_score <span class="built_in">float</span> <span class="keyword">generated</span> <span class="keyword">always</span> <span class="keyword">as</span> (chinese * math * english * <span class="number">10</span>) <span class="keyword">stored</span>;</span><br></pre></td></tr></table></figure>

<p>重命名生成的列</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">change</span> times_score times_score_new <span class="built_in">float</span> <span class="keyword">generated</span> <span class="keyword">always</span> <span class="keyword">as</span> (chinese * math * english * <span class="number">10</span>) <span class="keyword">stored</span>;</span><br></pre></td></tr></table></figure>

<p>删除生成的列</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">drop</span> <span class="keyword">column</span> times_score_new;</span><br></pre></td></tr></table></figure>

<p>virtual 列不能更改为 stored 的生成列，反之亦然。只能先删除，然后再重新添加</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">drop</span> <span class="keyword">column</span> total_score;</span><br><span class="line"></span><br><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">add</span> <span class="keyword">column</span> total_score <span class="keyword">double</span> <span class="keyword">generated</span> <span class="keyword">always</span> <span class="keyword">as</span> (chinese + math + english) <span class="keyword">stored</span>;</span><br></pre></td></tr></table></figure>

<p>表中的常规字段，可以修改为 stored 生成列，但不能是 virtual 生成列</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">modify</span> <span class="keyword">column</span> chinese <span class="keyword">double</span> <span class="keyword">generated</span> <span class="keyword">always</span> <span class="keyword">as</span> (math + <span class="number">1</span>) <span class="keyword">stored</span>;</span><br></pre></td></tr></table></figure>

<p>stored 生成列可以修改为常规字段，值为生成值</p>
<figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">alter</span> <span class="keyword">table</span> <span class="keyword">test</span> <span class="keyword">modify</span> <span class="keyword">column</span> total_score <span class="keyword">double</span>;</span><br></pre></td></tr></table></figure>
 
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        <div class="pswp__container">
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
            <div class="pswp__item"></div>
        </div>

        <!-- Default (PhotoSwipeUI_Default) interface on top of sliding area. Can be changed. -->
        <div class="pswp__ui pswp__ui--hidden">

            <div class="pswp__top-bar">

                <!--  Controls are self-explanatory. Order can be changed. -->

                <div class="pswp__counter"></div>

                <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>

                <button class="pswp__button pswp__button--share" style="display:none" title="Share"></button>

                <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                <!-- Preloader demo http://codepen.io/dimsemenov/pen/yyBWoR -->
                <!-- element will get class pswp__preloader--active when preloader is running -->
                <div class="pswp__preloader">
                    <div class="pswp__preloader__icn">
                        <div class="pswp__preloader__cut">
                            <div class="pswp__preloader__donut"></div>
                        </div>
                    </div>
                </div>
            </div>

            <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                <div class="pswp__share-tooltip"></div>
            </div>

            <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
            </button>

            <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
            </button>

            <div class="pswp__caption">
                <div class="pswp__caption__center"></div>
            </div>

        </div>

    </div>

</div>

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.css">
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/default-skin/default-skin.min.css">
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/photoswipe@4.1.3/dist/photoswipe-ui-default.min.js"></script>

<script>
    function viewer_init() {
        let pswpElement = document.querySelectorAll('.pswp')[0];
        let $imgArr = document.querySelectorAll(('.article-entry img:not(.reward-img)'))

        $imgArr.forEach(($em, i) => {
            $em.onclick = () => {
                // slider展开状态
                // todo: 这样不好，后面改成状态
                if (document.querySelector('.left-col.show')) return
                let items = []
                $imgArr.forEach(($em2, i2) => {
                    let img = $em2.getAttribute('data-idx', i2)
                    let src = $em2.getAttribute('data-target') || $em2.getAttribute('src')
                    let title = $em2.getAttribute('alt')
                    // 获得原图尺寸
                    const image = new Image()
                    image.src = src
                    items.push({
                        src: src,
                        w: image.width || $em2.width,
                        h: image.height || $em2.height,
                        title: title
                    })
                })
                var gallery = new PhotoSwipe(pswpElement, PhotoSwipeUI_Default, items, {
                    index: parseInt(i)
                });
                gallery.init()
            }
        })
    }
    viewer_init()
</script>

<!-- MathJax -->

<script type="text/x-mathjax-config">
  MathJax.Hub.Config({
      tex2jax: {
          inlineMath: [ ['$','$'], ["\\(","\\)"]  ],
          processEscapes: true,
          skipTags: ['script', 'noscript', 'style', 'textarea', 'pre', 'code']
      }
  });

  MathJax.Hub.Queue(function() {
      var all = MathJax.Hub.getAllJax(), i;
      for(i=0; i < all.length; i += 1) {
          all[i].SourceElement().parentNode.className += ' has-jax';
      }
  });
</script>

<script src="https://cdn.jsdelivr.net/npm/mathjax@2.7.6/unpacked/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<script>
  var ayerConfig = {
    mathjax: true
  }
</script>

<!-- Katex -->

<!-- busuanzi  -->


<script src="/js/busuanzi-2.3.pure.min.js"></script>


<!-- ClickLove -->

<!-- ClickBoom1 -->

<!-- ClickBoom2 -->

<!-- CodeCopy -->


<link rel="stylesheet" href="/css/clipboard.css">

<script src="https://cdn.jsdelivr.net/npm/clipboard@2/dist/clipboard.min.js"></script>
<script>
  function wait(callback, seconds) {
    var timelag = null;
    timelag = window.setTimeout(callback, seconds);
  }
  !function (e, t, a) {
    var initCopyCode = function(){
      var copyHtml = '';
      copyHtml += '<button class="btn-copy" data-clipboard-snippet="">';
      copyHtml += '<i class="ri-file-copy-2-line"></i><span>COPY</span>';
      copyHtml += '</button>';
      $(".highlight .code pre").before(copyHtml);
      $(".article pre code").before(copyHtml);
      var clipboard = new ClipboardJS('.btn-copy', {
        target: function(trigger) {
          return trigger.nextElementSibling;
        }
      });
      clipboard.on('success', function(e) {
        let $btn = $(e.trigger);
        $btn.addClass('copied');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-checkbox-circle-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPIED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-checkbox-circle-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
      clipboard.on('error', function(e) {
        e.clearSelection();
        let $btn = $(e.trigger);
        $btn.addClass('copy-failed');
        let $icon = $($btn.find('i'));
        $icon.removeClass('ri-file-copy-2-line');
        $icon.addClass('ri-time-line');
        let $span = $($btn.find('span'));
        $span[0].innerText = 'COPY FAILED';
        
        wait(function () { // 等待两秒钟后恢复
          $icon.removeClass('ri-time-line');
          $icon.addClass('ri-file-copy-2-line');
          $span[0].innerText = 'COPY';
        }, 2000);
      });
    }
    initCopyCode();
  }(window, document);
</script>


<!-- CanvasBackground -->


<script src="/js/dz.js"></script>



    
  </div>
</body>

</html>